1. Field of the Invention
This invention relates generally to carburizing methods and apparatus, and more particularly, to a control system for a multi-zone carburizing furnace of the push type that is capable of normal and suspend carburizing and capable of carburizing utilizing either an endothermic gas process or a nitrogen methanol process.
2. Description of the Prior Art
Multi-zone push type carburizing furnaces of are known. In such furnaces, trays of parts, typically fabricated from ferrous metals, are placed in a tray and "pushed" into a first zone of the carburizing furnace. The trays are kept in the first zone for a predetermined time, during which time a predetermined amount of carburizing takes place. After the expiration of the predetermined length of time, a second tray is pushed into the first zone, thereby advancing the first tray. The process is repeated until the first tray, and the trays subsequently pushed in are advanced through the various zones of the carburizing furnace and discharged at the opposite end. In such carburizing furnaces, the temperatures and the atmospheres of the various zones must be carefully controlled to maintain the desired temperature and carbon potential required for the particular carburizing being done.
In one type of prior art carburizing furnace, the temperature in each of the zones is controlled thermostatically with the thermostat which is either manually set or remotely set by means of some sort of control system. In such a furnace, the atmosphere is generally an endothermic gas atmosphere, which is generated by an endothermic gas generator. The endothermic gas is usually enriched by the addition of methane (CH.sub.4) or natural gas. In a typical endothermic gas generator, the endothermic gas is made by cracking methane with air to provide an endothermic gas composition of approximately 40% nitrogen, 40% hydrogen, 20% carbon monoxide, 0.1 to 0.5% carbon dioxide and 0.1 to 0.5 water vapor.
In an alternative method of generating the carburizing carrier gas, nitrogen is reacted with methanol to provide the carrier gas in the following equation: EQU 2N.sub.2 +CH.sub.3 OH.fwdarw.CO+2H.sub.2 +2N.sub.2
The above reaction provides a gas having a composition of approximately 40% nitrogen, 40% hydrogen and 20% carbon monoxide. This gas is also enriched by the addition of methane (CH.sub.4) or natural gas to provide the carburizing atmosphere.
However, push type carburizing furnaces operating in the normal carburizing mode have a basic disadvantage. This disadvantage relates to the length of time required for each tray to pass through the furnace, and results in a long start up time and a long shut down time for the carburizing furnace. For example, it may take on the order of four hours for a tray of parts to pass completely through the furnace from the first zone through the last. Consequently, when the carburizing operation is to be shut down for a period of time, such as a holiday period or a weekend, the operator cannot load any parts into the carburizing furnace for the last four hours of the shift prior to the shut down. Instead, empty trays are loaded into the carburizing furnace in order to push the last of the parts through the furnace before shut down. Production is lost during that four hour period. In addition, because of the large thermal mass of the furnace, several hours are required to bring the furnace up to temperature following the end of the weekend or the holiday period. Moreover, once the furnace has reached operating temperature, because of the time required for the parts to pass through the furnace, another four hours or so must elapse before the first parts are expelled from the furnace. Consequently, full production is obtained only during Tuesday through Thursday of a normal work week.
In an effort to overcome the disadvantages of the normal carburizing operation, a suspend carburizing operation has been developed. In a suspend carburizing operation, full trays are pushed through the furnace until just shortly before the end of the last shift prior to the weekend or holiday period, and the furnace is switched into a suspend mode of operation for the weekend. In the suspend mode of operation, the temperature of the furnace is reduced, typically from a normal carburizing temperature of on the order of 1700.degree. F. to a suspend carburizing temperature of on the order of approximately 1200.degree. F. to 1300.degree. F. During the suspend carburizing cycle, the carburizing gases are expelled, and the furnace is filled with an inert atmosphere, usually nitrogen. During this suspended mode of operation, the carburizing process is suspended; however, carburizing may readily be resumed by replacing the inert gas atmosphere with a carburizing atmosphere and raising the temperature to the normal carburizing temperature. The advantage of suspend carburizing over normal carburizing is that production can continue until almost the end of the last shift prior to the suspension. In addition, since the furnace is not completely cooled, the time required to bring the furnace up to temperature is substantially shorter, and more importantly, since the furnace is now full of parts that have been carburized to various degrees, the output of the carburized parts begins almost immediately after normal carburizing temperature has been reached.
However, one of the disadvantages of suspend carburizing is that during the transition from the carburizing to the suspend mode, the composition of the carburizing atmosphere must be changed to reflect the change in the carbon potential as a function of temperature during the ramp down of temperature to the suspend mode. Moreover, the amount of carburizing that results during the ramp down of temperature, as well as the carburizing that occurs during the ramp up in temperature following the suspend period must be calculated, and the remainder of the carburizing process must be adjusted to account for this carburizing that took place during the ramp up cycle and ramp down cycle. Thus, in addition to accurate temperature control, the flow as well as the composition of the carburizing atmosphere must be accurately controlled. These requirements make it advantageous to utilize a computer controlled control system to control the carburizing process particularly during the transition from normal to suspend carburizing and vice versa.
Although it is possible to control the temperatures of the various zones in a carburizing furnace by means of a microprocessor and appropriate temperature sensing equipment, the control of the carburizing atmosphere is much more difficult, particularly when an endothermic gas generator is used. One reason for the difficulty in controlling the composition of the atmosphere is that an endothermic gas generator is a device that generates the endothermic gas in a process that operates at a substantially constant volume, temperature and input gas flow, and serves to provide an endothermic gas having substantially constant properties. Any attempt to change the characteristics of the endothermic output gas requires a change in the reaction occurring in the endothermic gas generator. Unfortunately, such changes are not made readily, and the results of such changes are unpredictable.
The problems associated with the control of the carburizing atmosphere are largely alleviated by utilizing the nitrogen-methanol method of generating the carrier gas. In the nitrogen-methanol method of generating the carrier gas, the reaction that forms the carrier gas occurs inside the carburizing furnace, rather than in an external generator. Consequently, the composition of the carrier gas can be readily controlled by simply controlling the amount of nitrogen (in gas form) and methanol (in liquid form) that is injected into the furnace. Unfortunately, a drawback of the nitrogen-methanol process is cost, and the increased cost of generating the carrier gas by the nitrogen-methanol process nullifies much of the cost advantages obtained from the substantially continuous production that can be obtained from a suspend carburizing process.
Accordingly, it is an object of the present invention to provide an improved carburizing method and apparatus that overcomes many of the disadvantages of the prior art.
It is another object of the present invention to provide a new carburizing method and apparatus that substantially reduces the cost of carburizing.
It is yet another object of the present invention to provide an improved carburizing method and apparatus that is capable of doing both normal and suspend carburizing which generates the carrier gas with either an endothermic gas generator or by the nitrogen-methanol process in order to optimize carburizing efficiency.
It is still another object of the present invention to reduce the cost of producing carrier gas by the nitrogen-methanol process through the use of high and low carrier gas flow rates during various stages of the carburizing process.
It is yet another object of the present invention to improve the control of the composition of the carrier gas produced by the nitrogen-methanol method, particularly during low flow rates, by using a multi-stage cascaded valving system.